The present invention relates generally to x-ray bone densitometers for measuring bone health and particularly a densitometer adapted to monitor changes in bone health of a patient over a period of time.
X-ray bone densitometers make measurements at two x-ray energies to provide separate attenuation images of two basis materials, typically bone and soft tissue. The bone attenuation image is substantially free from attenuation caused by soft tissue allowing areal bone density (g/cm2) to be accurately determined in vivo for assessments of bone strength and health. The bone attenuation image also provides improved definition of bone outlines, allowing measurements, for example, of bone morphology (e.g., vertebral height) such as may be useful for detecting crush fractures associated with osteoporosis.
Normally, such measurements evaluate a bone density within a region of interest (ROI) located within a bone (typically the neck of the femur or body of lower vertebrae) as referenced to one or more landmarks on the bone.
Often it is desired to detect changes in particular bones over time or over the course of a treatment. Positioning errors caused by changes in the position of the patient with respect to the densitometer can affect measurements of bone density in an ROI, by changing the apparent location of the landmarks used to locate the ROI and/or by changing the apparent density of the bone within the ROI by foreshortening caused by bone rotation.
It may be desired to evaluate localized changes in bone density, for example, in subregions distributed about the bone to detect subtle changes obscured when average bone density in a large area is examined. Detailed comparison of subregions of bone are also hampered by positioning errors which prevent direct comparison of bone images taken at different times.